Manufacture of fluorescent lamps and screens



' In carrying out the invention, assuming Patented Jan. 11, 1944 MANUFACTURE OF FLUORESCENT LAMPS AND SCREENS James Thomson Anderson, Hillmorton, Rugby, 1 England, assignor to General Electric (Jompany, a corporation of New York I No Drawing. Application August 25, 1941, Serial 9 Claims.

My invention relates to methods of applying finely divided materials, such as luminescent powders, to supporting surfaces.

In the manufacture of tubular fluorescent lampsespecially those in which to achieve a "desired fluorescent color, more than one fluorescent powder is used-it is a recognized procedure to mix the powders as-a suspension in a viscous fluid such as a solution of nitrocellulose. The

inside of the tube is then coated with this susders.

An object of the invention is to provide an improved method whereby, when a binder such as nitrocellulose is used as a vehicle for carrying the powders on to the glass, the binder may then be partly or wholly removed without removing any substantial amount of the powders. In accordance with my invention, this is accomplished by the action of solvent on the binder to soften, dissolve, and remove it. Other objects and advantages of the invention will appear from the following detailed description thereof.

According to the invention as hereinafter described, the binder holding a thin layer of a powdered substance on the surface of a foundation body or support is removed by first treating the binder with the vapor of a solvent for the binder, and then removing the treated binder by the application of the solvent in liquid form.

I :believe that the vapor softens the binder, allows the powder to reach the surface of the foundation and that the softened binder is then removed by the liquid solvent.

trooellulose constitutes the binder, and acetori the solvent, the vapor of the solvent is applied to the surface to which the powder has been caused to adhere, by bubbling a stream of air through a bottle of acetone, and then through a tube containins or constituting the foundation body until the binder is in such a condition that it can be In Great Britain October 14,

instance it can be achieved by simply inserting a little acetone into the bottom of the tube and allowing the latter to stand for some hours so that the acetone vapor diifuses throughout the air in the tube. This generally requires about 12-16 hours unless the tube is emptied (after about 4 hours), dried, inverted and some acetone inserted at the other end for a similar time.

It will be realized that the time of softening (i. e. treatment with solvent vapor) and the time of soaking (treatment with liquid solvent) depend upon thickness, solubility, etc. of the nitrocellulose, and on the rate of air flow, temperature, etc., but I have found that one to two hours treatment with acetone saturated air at approximately 20 C. at an approximate rate of 10 litres per hour is sufficient for the softening. Most of the nitrocellulose is then removed by treating with liquid acetonefor about two hours,though H more or less time may be needed depending upon whether the acetone already contains some solute, and upon whether the acetone is allowed to stand or to circulate either within the tube itself or within a system of which the tube is a part.

It is obvious, of course, that not more than'a few minutes should elapse between the end of the removed by application of the liquid solvent,

i. e. by filling the tube with the liquid and then allowing it to run out. a The method of "softening" is not critical, for

softening and the beginning of the soaking process and in practice it is found that 2 minutes is a suitable maximum time to allow.

The vapors of other substances which dissolve the nitrocellulose can be used instead of acetone vapor. Other solvents for the nitrocellulose than acetone may be used for the removal of the binder after the vapor treatment. For example, amyl, butyl and ethyl acetate, or a mixture of alcohol and ether are useful as solvents for the nitrocellulose in the softening and soaking" steps of the method. Ethylcellulose may be used as an organic binder, instead of nitrocellulose, when desired, and this is treated with butyl alcohol or acetone in accordance with the method of the present invention.

Advantages of my method are that the powder is not subjected to more than a fraction of the usual contact with decompositionproducts of 'the binder, thus giving better brilliance and maintenance in the finished lamp or screen.

Also, the time of baking to remove the binder may be considerably shortened or ,even omitted from the manufacturing schedule, thus minimizing or eliminating the risk of distorting the shape of the tube, and of diminishing the brilliance of the powder by the baking.

The method described above is useful in'the prising soluble binding material, drying the coating on the surface, softening the dried binder material in the coating with a solvent thereof, and dissolving out and removing the softened binder material with a liquid solvent thereof, leaving the aforesaid finely divided material on the supporting surface.

7 2. The method of coating a fluorescent lamp envelope with luminescent powder particles which comprises coating the envelope surface with a liquid suspension of the powder in a viscous binder, drying the coating on the surface, softening the dried binder material in the coating with a solvent for the binder, and thus allowing the luminescent particles themselves to reach said surface, and dissolving out and removing the softened binder material with a liquid solvent thereof while leaving the luminescent particles on the surfaceundisturbed.

3. The method of removing a binder holding prises the steps of treating the binder with solvent for said binder in the vapor phase and with solvent for said binder in the liquid phase, and removing the liquid with its dissolved binder material, leaving the powder on the support.

4. The method of removing a binder holding powder on the inner surface of a hollow support which comprises the steps of filling said support with air saturated with a solvent for the binder powder on the surface of a support which comto soften the latter, then filling the support with powder on the inner surface of a hollow support which comprises the steps of passing through the support air saturated with a solvent for the binder to soften the latter, then filling the support with liquid solvent for said binder and then draining the liquid from the support to remove the binder and to leave the powder on the support. a

6. The method of removing a binder holding powder on the inner surface of a hollow support which comprises the steps of filling said support with air saturated with a solvent for the binder to soften 'the latter, then circulating liquid solvent for said binder in contact with the binder, and then draining the liquid from the support to remove the binder and to leave the powder on the support.

7. The method of removing a nitrocellulose binder holding powder on the inner surface of a hollow support which comprises the steps of bringing the vapor of a solvent for said binder into contact with the binder to soften the latter, then bringing liquid solvent for said binder into contact with the binder, and then draining off the liquid to remove the binder and to leave the powder on the support.

8. The method of removing a nitrocellulose binder holding fluorescent powder on the inner ,surface of a, vitreous tube which comprises the steps of bringing acetone vapor into contact with the nitrocellulose binder to soften the latter, then bringing liquid acetone into contact with the softened nitrocellulose, and then draining the liquid from the support to remove the binder and to leave the fluorescent powder on the support.

9. The method of removing an ethylcellulose binder holding powder on the inner surface of a hollow support which comprises the steps of bringing the vapor of a solvent for said binder into contact with the binder to soften the latter, then bringing liquid solvent for said binder into contact with the binder, and then draining of! the liquid to remove the binder and to leave the powder on the support.

JAMES THOMSON ANDERSON. 

